I'm aware of V-Slot connector but when I printed it, I had trouble getting it sufficiently precise. So instead I milled H-shaped connectors from UHMW PE stock, and found that a 6mm "waist" in the H and a 3.58mm wide channel appears perfect; it holds the pieces firmly and precisely together; any narrower and it's too hard to install, wider and it's a sloppy fit that doesn't quite accomplish the goal. I suspect that the exact dimensions for the channel width would be different in acetal but I haven't experimented with it. My outside dimensions were 10mm x 6.8mm based on measurements of v-slot I have. FreeCAD made a hash of exporting the quick hack ugly drawing to SVG, but you can download it to clarify which dimensions are which. The PDF is better. Here's the preview:
